1. Field of the Invention
The present invention relates to a switching DC power supply apparatus having an overcurrent protective circuit for realizing optimal overcurrent protection, wherein the ability to supply peak currents is required more than the ability to supply average currents as in the case of current supply to a motor load or the like.
2. Related Background Art
As DC power supply apparatuses for various types of electronic devices, switching DC power supply apparatuses have been widely used because they are compact, lightweight, and highly efficient.
FIG. 5 shows the arrangement of a conventional switching DC power supply apparatus having an output stabilizing circuit.
Referring to FIG. 5 , the apparatus includes input power supply terminals 1 and 2. The input power supply terminal 1 is connected to an input capacitor C1 and a primary winding N1 of a transformer T1.
One end of the primary winding N1 is connected to the collector of a switching transistor Q1. A secondary winding N2 of the transformer T1 is connected to a smoothing capacitor C2 through a rectifying diode Q3. A smoothed output obtained by this filter circuit is supplied to a predetermined load through output terminals 4 and 5.
In order to detect an output voltage applied to the load, a voltage detection circuit 6 is connected in parallel with the load (not shown) connected to the terminals 4 and 5. A detected voltage value is fed back to a control circuit constituted by a PWM circuit 3 through a photocoupler Q4.
The photocoupler Q4 output the fed-back detected voltage to the PWM circuit 3. The PWM circuit 3 controls the switching duty ratio or frequency of a driving pulse supplied to the base of the switching transistor Q1 in accordance with the detected voltage value.
The switching transistor Q1 is driven by an output from the PWM circuit 3.
For a protective function, a current flowing in the switching transistor Q1 is detected by a primary current detection means A constituted by a comparator Q2, a detection resistor R1, and the like.
An overcurrent protective operation in the circuit shown in FIG. 5 will be described next.
When a power supply is connected to the input power supply terminals 1 and 2, voltage application to the primary winding N1 of the transformer T1 is ON/OFF-controlled by the switching transistor Q1.
The AC output generated by the ON/OFF operation of the switching transistor Q1 is insulated and transformed by the transformer T1.
The insulated AC output is rectified by the diode Q3. The rectified output is smoothed by the smoothing capacitor C2 and is subsequently supplied to the load through the output terminals 4 and 5.
If a larger load is set and a larger current is output from the switching transistor Q1, a higher voltage is generated across the detection resistor R1. When this voltage exceeds the threshold value of the comparator Q2, a signal is output to the PWM circuit 3. In response to the signal from the comparator Q2, the PWM circuit 3 stops a pulse output operation. This operation is performed for every pulse.
Resistors R2 and R3, and a capacitor C3 constituting the primary current detection means A serve as a filter circuit for preventing an operation error due to spike noise, whereas resistors R4 and R5 serve to set a threshold value.
According to such a conventional apparatus, however, in a power supply for supplying more peak load currents than average currents as in the case of current supply to a motor load, since the detection current value of the primary current detection means A is set in accordance with the value of the peak load current, if an overload state continues because of a load error or the like, the power supply is continuously operated with the maximum current, resulting in overheating. Furthermore, in order to prevent such overheating, the heat capacity of a heat sink plate must be increased or an overheat protective circuit must be added. This poses other problems, e.g., an increase in size and cost of the power supply.
The apparatus shown in FIG. 5 has only one output system. FIG. 6 is a circuit diagram showing a conventional switching DC power supply apparatus having two output systems.
Referring to FIG. 6, the apparatus includes input power supply terminals 1' and 2', which are connected to an input capacitor C1' and are further connected to a primary winding N1' of a transformer T1' through a resistor R1' and a switching transistor Q1'.
A secondary winding N2' of the transformer T1' is connected to a smoothing capacitor C2' through a rectifying diode Q3'. The smoothed output obtained this filter circuit is supplied to a first load through output terminals 4' and 5'.
Similarly, a secondary winding N3' is connected to a smoothing capacitor C3' through a rectifying diode Q4'. The smoothed output obtained by this filter circuit is supplied to a second load through output terminals 6' and 7'.
In order to detect an output voltage applied to the first load, an output voltage detection circuit 8' is connected in parallel with the first load (not shown) connected to the output terminals 4' and 5'. The detected value is fed back to a control circuit constituted by a PWM (pulse width modulation) circuit 3' through a photocoupler Q5'.
The PWM circuit 3' controls the switching duty ratio or frequency of a driving pulse supplied to the base of the switching transistor Q1' in accordance with the detected value. In this manner, the switching transistor Q1' is driven by an output from the PWM circuit 3', and the voltage across the output terminals 4' and 5' is controlled to be a predetermined value.
For a protective function, a voltage proportional to the instantaneous value of a switching current flowing in the switching transistor Q1 is obtained as a detected voltage by the resistor R1'. Resistors R2' and R3' constitute a resistance voltage dividing circuit for dividing the detected voltage obtained by the resistor R1' into proper values.
Resistors R4' and R5' constitute a resistance voltage dividing circuit for setting a threshold voltage for overload detection. A comparator Q2' of a comparison circuit serves to compare a detected voltage with a threshold voltage.
An overload protective operation in the circuit shown in FIG. 6 will be described below.
When a DC power supply is connected to the input power supply terminals 1' and 2', voltage application to the primary winding N1' of the transformer T1' is ON/OFF-controlled by the switching transistor Q1'. The AC output generated by the ON/OFF operation of the switching transistor Q1' is insulated and transformed by the transformer T1'.
The insulated and transformed AC output is rectified by the diode Q3'. The rectified output is smoothed by the smoothing capacitor C2' and is subsequently supplied to the first load through the output terminals 4' and 5'.
Similarly, the rectified output obtained by the diode Q4' is smoothed by the smoothing capacitor C3' and is subsequently supplied to the second load through the output terminals 6' and 7'.
If a larger load is set and a larger current is output from the switching transistor Q1', a higher voltage is generated across the resistor R1'. When this voltage exceeds the threshold value, the comparator Q2' outputs a signal to the PWM circuit 3'. In response to the signal from the comparator Q2', the PWM circuit 3' stops a pulse output operation to perform overload protection. This operation is performed for every pulse.
In the conventional apparatus shown in FIG. 6, however, overload protection is performed by limiting the sum of power (to be referred to as total power hereinafter) of all the output systems.
If, therefore, a load is concentrated on a specific secondary circuit, since power can be supplied up to the total power value, a specific part may be overheated, or an overcurrent may flow in the load, resulting in burning the load.